DESCRIPTION: The long-term goal of this revised research proposal is to determine the specific mechanisms by which progesterone mediates neuronal rescue and recovery of function in traumatic brain injury (TBI); the major cause of death in young adults under the age of 35. These findings will be employed in the evaluation of progesterone as a safe and effective treatment for TBI. Progesterone has recently been classified as a neurosteroid because it is also synthesized in astrocytes and oligodendrocytes in the brains of both males and female, where it may play other roles than that of a sex hormone. Previous research has already shown that systemic injection of progesterone in laboratory rats can reduce some of the neuropathological consequences of TBI and enhance behavioral recovery of function. Progesterone treatments reduce cerebral edema dramatically in both males and females when administered within 24 hours of the injury. This is one type of protective effect that can lead to reduced neuronal death and improved functional outcomes. The primary goal of the current proposal is to understand better the physiological mechanisms by which progesterone mediates its neuroprotective actions. We propose a series of 4 Aims using both in vitro and in vivo models of neural injury to determine: (1) if progesterone exerts its effects via its specific receptors. This will be studied by co-administering the hormone with ORG31710, a known progesterone receptor antagonist, or substituting progesterone with an agonist that cannot be metabolized (R5020) in both in vivo and in vitro models of TBI; (2) if, in an in vitro model of injury, progesterone's short-term neuroprotective effects are mediated specifically via sigma receptors; (3) if progesterone will prevent loss of mitochondrial function by reducing oxidative stress initiated in both in vivo and in vitro models of TBI; and (4) if progesterone will reduce the destructive aspects of the inflammatory immune reaction that occurs after TBI. The combination of in vivo and in vitro models will provide parallel evaluation of the mechanisms of progesterone's action. The research proposed here will help to determine whether progesterone can be used as a low-cost, safe and effective therapeutic agent in the acute stages of CNS injury. Furthermore, the detailed assessment of progesterone's mechanisms of action will provide a foundation for the intelligent design of artificial therapeutic pharmaceuticals.